Branch timing

Branches are one of the instruction types that do not always take one cycle. The only type of branch that does take one cycle is a conditional branch that is not taken. Branches that are taken take two cycles and branching to an unaligned wide instruction takes three cycles. Examples of starting a loop on an unaligned instruction can be seen in the examples below. The table shows the performance impact, while both functions jump twice, and a CPI count of two is expected, sample a uses 3 additional CPI cycles due to starting the loop with an unaligned wide instruction.

Examples of unaligned branching to wide and narrow instructions.

power:
    mov r2, r0
    mov r0, #1     
    loop:
        mul r0, r2 //02fb00f0
        sub r1, #1
        cmp r1, #0
        bne loop
    bx lr

multiply:
    mov r2, r0
    mov r0, #1     
    loop:
        add r0, r2 //1044
        sub r1, #1
        cmp r1, #0
        bne loop
    bx lr

Performance comparison of unaligned loops. Note that 2 and 3 were passed as arguments.